A molecular dynamics simulation of a membrane with net charge in its liquid-crystalline state was carried out. It was modeled by dipalmitoylphosphatidylserine lipids with net charge, sodium ions as counterions and water molecules. The behavior of this membrane differs from that was shown by other membranes without a net charge as a consequence of strong Coulomb interaction between atoms of adjacent phospholipids. The most remarkable effect produced by such interaction between neighboring lipids is a reduction of the surface area per phospholipid compared to an uncharged membrane. In addition, other properties of the membrane were also affected by this interaction between adjacent lipids such as the atom distribution across the membrane, the diffusion coefficient of the different components of the membrane and the order parameter of the phospholipid hydrocarbon region. Some comparisons of this membrane with dipalmitoylphosphatidylcholine membrane without net charge at similar conditions are presented.

The first stage of the action mechanism of small cationic peptides with antimicrobial activity is ruled by electrostatic interactions between the peptide and the pathogen cell membrane. Thus, an increase in its activity could be expected with an increase in the positive charge on the peptide. By contrast, the opposite behavior has been observed when the charge increases to reach a critical value, beyond which the activity falls. This work studies the perturbation effects in a cell membrane model for two small cationic peptides with similar length and morphology but with different cationic charges. The synthesis and antibacterial activity of the two peptides used in this study are described. The thermodynamic study associated with the insertion of these peptides into the membrane and the perturbing effects on the bilayer structure provide valuable insights into the molecular action mechanism associated with the charge of these small cationic peptides.

Pathogenic bacteria have the ability to develop antibiotic resistance mechanisms. Their action consists mainly in the production of bacterial enzymes that inactivate antibiotics or the appearance of modifications that prevent the arrival of the drug at the target point or the alteration of the target point itself, becoming a growing problem for health systems. Chitosan–gold nanoparticles (Cs-AuNPs) have been shown as effective bactericidal materials avoiding damage to human cells. In this work, Cs-AuNPs were synthesized using chitosan as the reducing agent, and a systematic analysis of the influence of the synthesis parameters on the size and zeta potential of the Cs-AuNPs and their UV-vis spectra was carried out. We used a simulation model to characterize the interaction of chitosan with bacterial membranes, using a symmetric charged bilayer and two different chitosan models with different degrees of the chitosan amine protonation as a function of pH, with the aim to elucidate the antibacterial mechanism involving the cell wall disruption. The Cs-AuNP antibacterial activity was evaluated to check the simulation model.

A molecular dynamic simulation of wet polypyrrole film was carried out, in both oxidized and reduced state. The system was modeled by two layers of polypyrrole, water and chloride ions (as counterions required for charge balance in the oxidized state) in atomic detail to provide an insight into some dynamic and steady properties of the system. Our simulations pointed to a swelling of the polymer matrix after oxidation due to electrostatic repulsions between charged sites of the oxidized polypyrrole, followed by penetration of the polypyrrole by counterions to maintain the electroneutrality of the system. Associated with this penetration of counterions toward the core of the oxidized polypyrrole, dehydration of the counterions was observed. This dehydration was compensated (in part) by a strong coordination with the charged sites of the polymer. The remaining hydrophobicity inside the polymer also contributed to the dehydration of these counterions. The translational diffusion coefficient of chloride ions was also calculated at different positions of the polypyrrole/water interface, from bulk water to the inner polymer matrix. A value of ${\text{4.1×10}}^{\text{−5}}\hspace{0.17em}{\mathrm{cm}}^2 {\mathrm{s}}^{\text{−1}}$ was measured in the bulk water compared to ${\text{5×10}}^{\text{−7}}\hspace{0.17em}{\mathrm{cm}}^2 {\mathrm{s}}^{\text{−1}}$ inside the polymer, representing a diminution of two orders of magnitude for the translational diffusion coefficient from bulk water to the core of a oxidized polypyrrole matrix. These results were in good agreement with experimental data.